1. Field of the Invention
The present invention is directed generally to wireless communication systems and, more particularly, to the transmission of physical downlink control signaling.
2. Description of the Art
A communication system includes a DownLink (DL) that conveys transmission signals from transmission points, such as Base Stations (BS) or NodeBs to User Equipments (UEs), and an UpLink (UL) that conveys transmission signals from UEs to reception points such as the NodeBs. A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a cellular phone, a personal computer device, etc. A NodeB is generally a fixed station and may also be referred to as an access point or some other equivalent terminology.
DL signals include data signals carrying information content, control signals, and Reference Signals (RSs), which are also known as pilot signals. A NodeB conveys data signals to UEs through Physical Downlink Shared CHannels (PDSCHs) and control signals to UEs through Physical Downlink Control CHannels (PDCCHs). UL signals also include data signals, control signals, and RS. UEs convey data signals to NodeBs through Physical Uplink Shared CHannels (PUSCHs) and control signals to NodeBs through Physical Uplink Control CHannels (PUCCHs). It is possible for a UE having transmission of data information to also convey control information through the PUSCH.
Downlink Control Information (DCI) serves several purposes and is conveyed through DCI formats transmitted in PDCCHs. For example, DCI includes DL Scheduling Assignments (SAs) for PDSCH reception and UL SAs for PUSCH transmission. Because PDCCHs are a major part of a total DL overhead, their resource requirements directly impact the DL throughput. One method for reducing PDCCH overhead is to scale its size according to the resources required to transmit the DCI formats during a DL Transmission Time Interval (TTI). Assuming Orthogonal Frequency Division Multiple (OFDM) as the DL transmission method, a Control Channel Format Indicator (CCFI) parameter transmitted through the Physical Control Format Indicator CHannel (PCFICH) can be used to indicate the number of OFDM symbols occupied by the PDCCHs in a DL TTI.
FIG. 1 illustrates a conventional structure for PDCCH transmissions in a DL TTI.
Referring to FIG. 1, a DL TTI is assumed to consist of one subframe having N=14 OFDM symbols. A DL control region including the PDCCH transmissions occupies a first M OFDM symbols 110, i.e., M=3. A remaining N-M OFDM symbols are used primarily for PDSCH transmissions 120, i.e., M−N=9. A PCFICH 130 is transmitted in some sub-carriers, also referred to as Resource Elements (REs), of a first OFDM symbol and includes 2 bits indicating a DL control region size, e.g., M=1, M=2, or M=3 OFDM symbols.
For two NodeB transmitter antennas, some OFDM symbols also include respective RS REs 140 and 150. These RSs are transmitted substantially over an entire DL operating BandWidth (BW) and are referred to as Common RSs (CRSs) as they can be used by each UE to obtain a channel estimate for its DL channel medium and to perform other measurements. Herein, a PDCCH transmitted with the conventional structure illustrated in FIG. 1 will be referred to as a cPDCCH.
Additional control channels may be transmitted in a DL control region, but they are not shown for brevity. For example, assuming the use of a Hybrid Automatic Repeat reQuest (HARQ) process for data transmission in a PUSCH, a NodeB may transmit a Physical Hybrid-HARQ Indicator CHannel (PHICH) to indicate to UEs whether or not their previous PUSCH transmissions were correctly received.
FIG. 2 illustrates a conventional encoding process for a DCI format.
Referring to FIG. 2, a NodeB separately codes and transmits each DCI format in a respective PDCCH. A Radio Network Temporary Identifier (RNTI) for a UE for which a DCI format is intended masks the Cyclic Redundancy Check (CRC) of a DCI format codeword in order to enable the UE to identify that the particular DCI format is intended for it. For example, both the CRC and the RNTI have 16 bits. The CRC 220 of the (non-coded) DCI format bits 210 is computed and it is subsequently masked 230 using the exclusive OR (XOR) operation between the CRC and RNTI bits 240. Accordingly, XOR(0,0)=0, XOR(0,1)=1, XOR(1,0)=1, and XOR(1,1)=0.
Thereafter, the masked CRC is appended to the DCI format information bits 250, channel coding is performed 260, e.g., using a convolutional code, and rate matching 270 is performed to the allocated resources. Interleaving and modulation 280 is performed, and a control signal 290 then transmitted.
FIG. 3 illustrates a conventional decoding process for a DCI format.
Referring to FIG. 3, a UE receiver performs the reverse operations of a NodeB transmitter to determine if the UE has a DCI format assignment in a DL subframe.
Specifically, a received control signal 310 is demodulated and the resulting bits are de-interleaved 320, a rate matching applied in a NodeB transmitter is restored 330, and data is subsequently decoded 340. After decoding, DCI format information bits 360 are obtained after extracting CRC bits 350, which are then de-masked 370 by applying the XOR operation with a UE RNTI 380. Finally, a UE performs a CRC test 390. If the CRC test passes, a UE considers a DCI format to be valid and determines parameters for signal reception or signal transmission. If the CRC test does not pass, a UE disregards the DCI format.
The DCI format information bits correspond to several fields, or Information Elements (IEs), e.g., the Resource Allocation (RA) IE indicating the part of the operating BandWidth (BW) allocated to a UE for PDSCH reception or PUSCH transmission, the Modulation and Coding Scheme (MCS) IE indicating the data MCS, the IE related to the HARQ operation, etc. The BW unit for PDSCH or PUSCH transmissions is assumed to consist of several REs, e.g., 12 REs, and will be referred to herein as a Resource Block (RB). Additionally, a RB over one subframe will be referred to as a Physical RB (PRB).
To avoid a cPDCCH transmission to a UE blocking a cPDCCH transmission to another UE, the location of each cPDCCH transmission in the time-frequency domain of a DL control region is not unique and, as a consequence, each UE performs multiple decoding operations to determine whether there are cPDCCHs intended for it in a DL subframe. The REs carrying each cPDCCH are grouped into conventional Control Channel Elements (cCCEs) in the logical domain. For a given number of DCI format bits in FIG. 2, the number of cCCEs for a respective cPDCCH depends on a channel coding rate (Quadrature Phase Shift Keying (QPSK) is assumed as the modulation scheme). A NodeB may use a lower channel coding rate and more cCCEs for cPDCCH transmission to UEs experiencing low DL Signal-to-Interference and Noise Ratio (SINR) than to UEs experiencing a high DL SINR. The cCCE aggregation levels include, for example, 1, 2, 4, and 8 cCCEs.
For a cPDCCH decoding process, a UE may determine a search space for candidate cPDCCH transmissions after restoring the cCCEs in the logical domain according to a common set of cCCEs for all UEs (UE-Common Search Space or UE-CSS) and according to a UE-dedicated set of cCCEs (UE-Dedicated Search Space or UE-DSS). For example, the UE-CSS includes the first C cCCEs in the logical domain. The UE-DSS may be determined according to a pseudo-random function having as inputs UE-common parameters, such as the subframe number or the total number of cCCEs in the subframe, and UE-specific parameters such as the RNTI. For example, for cCCE aggregation levels L∈{1,2,4,8}, the cCCEs corresponding to cPDCCH candidate m are given by Equation (1).cCCEs for cPDCCH candidate m=L·{(Yk+m)mod└NCCE,k/L┘}+i  (1)
In Equation (1), NCCE,k is the total number of cCCEs in subframe k, i=0, . . . , L−1, m=0, . . . , MC(L)−1, and MC(L) is the number of cPDCCH candidates to monitor in the search space. Exemplary values of MC(L) for L∈{1,2,4,8} are {6, 6, 2, 2}, respectively. For the UE-CSS, Yk=0. For the UE-DSS, Yk=(A·Yk−1) mod D, where Y−1=RNTI≠0, A=39827, and D=65537.
DCI formats conveying information to multiple UEs are transmitted in a UE-CSS. Additionally, if enough cCCEs remain after the transmission of DCI formats conveying information to multiple UEs, a UE-CSS may also convey some DCI formats for DL SAs or UL SAs. A UE-DSS exclusively conveys DCI formats for DL SAs or UL SAs. For example, a UE-CSS may include 16 cCCEs and support 2 DCI formats with L=8 cCCEs, 4 DCI formats with L=4 cCCEs, 1 DCI format with L=8 cCCEs, or 2 DCI formats with L=4 cCCEs. The cCCEs for a UE-CSS are placed first in the logical domain (prior to interleaving).
FIG. 4 illustrates a conventional transmission process for cPDCCHs.
Referring to FIG. 4, after channel coding and rate matching, as illustrated in FIG. 2, the encoded DCI format bits are mapped, in the logical domain, to cCCEs 400 of a cPDCCH. The first 4 cCCEs (L=4), i.e., cCCE1 401, cCCE2 402, cCCE3 403, and cCCE4 404, are used for cPDCCH transmission to UE1. The next 2 cCCEs (L=2), i.e., cCCE5 411 and cCCE6 412, are used for cPDCCH transmission to UE2. The next 2 cCCEs (L=2), i.e., cCCE7 421 and cCCE8 422, are used for cPDCCH transmission to UE3. Finally, the last cCCE (L=1), i.e., cCCE9 431, is used for cPDCCH transmission to UE4.
The DCI format bits are scrambled by a binary scrambling code in step 440 and are subsequently modulated in step 450. Each cCCE is further divided into mini-cCCEs or Resource Element Groups (REGs). For example, a cCCE including 36 REs can be divided into 9 REGs, each having 4 REs. Interleaving is applied among REGs (blocks of 4 QPSK symbols) in step 460. For example, a block interleaver may be used where interleaving is performed on symbol-quadruplets (4 QPSK symbols corresponding to the 4 REs of a REG) instead of on individual bits.
After interleaving the REGs, a resulting series of QPSK symbols may be shifted by J symbols in step 470, and finally, each QPSK symbol is mapped to an RE in a DL control region in step 480. Therefore, in addition to RSs 491 and 492 from NodeB transmitter antennas, and other control channels such as a PCFICH 493 and a PHICH (not shown), REs in a DL control region include QPSK symbols for cPDCCHs corresponding to DCI formats for UE1 494, UE2 495, UE3 496, and UE4 497.
A UE may transmit an ACKnowledgement signal associated with a HARQ process (HARQ-ACK signal) in a PUCCH in response to a reception of one or more data Transport Blocks (TBs) in a PDSCH. When a PDSCH is scheduled by a DL SA in a respective cPDCCH, a UE may implicitly derive a PUCCH resource nPUCCH for a HARQ-ACK signal transmission from the index of a first cCCE, nCCE, of a respective cPDCCH transmission. Therefore, for a PDSCH reception in a given DL subframe, a UE determines a PUCCH resource for an associated HARQ-ACK signal transmission in a subsequent UL subframe as nPUCCH=f(nCCE), where f( ) is a function providing a one-to-one mapping between a cCCE number and a PUCCH resource.
For example, f(nCCE)=nCCE+NPUCCH, where NPUCCH is an offset a NodeB informs to UEs by Radio Resource Control (RRC) signaling. If a UE is to determine multiple PUCCH resources for HARQ-ACK signal transmission, resources associated with several consecutive cCCEs after a first cCCE of a respective cPDCCH are used. For example, a second PUCCH resource may be obtained from f(nCCE+1). A UE can determine the total number of cCCEs used to transmit cPDCCHs in a subframe after decoding the PCFICH as, for a predetermined configuration of CRS REs, PHICH REs, and PCFICH REs, the number of cCCEs can be uniquely determined from the number of respective OFDM symbols.
The cPDCCH structure illustrated in FIG. 4 uses a maximum of M=3 OFDM symbols and transmits a control signal over an operating DL BW. Consequently, THE cPDCCH structure has limited capacity and cannot achieve interference co-ordination in the frequency domain.
There are several cases in which expanded capacity or interference co-ordination in the frequency domain is used for PDCCH transmissions. One such case is a communication system with cell aggregation, where the DL SAs or UL SAs to UEs in multiple cells are transmitted in a single cell (for example, because other cells may convey only PDSCH). Another case is extensive use of spatial multiplexing for PDSCH transmissions where multiple DL SAs correspond to same PDSCH resources. Another case is when DL transmissions from a first NodeB experience strong interference from DL transmissions from a second NodeB and DL interference co-ordination in the frequency domain between the two cells is needed.
A direct extension of a maximum DL control region size to more than M=3 OFDM symbols is not possible at least due to the requirement to support UEs which cannot be aware of such extension. Accordingly, a conventional alternative is to extend a DL control region in a PDSCH region and use individual PRBs for transmissions of control signals. Herein, a PDCCH transmitted in this manner will be referred to as enhanced PDCCH (ePDCCH).
FIG. 5 illustrates a conventional use of PRBs for ePDCCH transmissions in a DL TTI.
Referring to FIG. 5, although ePDCCH transmissions start immediately after cPDCCH transmissions 510 and are over all remaining DL subframe symbols, alternatively, they may start at a fixed location, such as the fourth OFDM symbol, and extend over a part of remaining DL subframe symbols. The ePDCCH transmissions occurs in four PRBs, 520, 530, 540, and 550, while remaining PRBs may be used for PDSCH transmissions 560, 562, 564, 566, and 568.
An ePDCCH reception may be based on a CRS or on a DemoDulation RS (DMRS). The DMRS is UE-specific and is transmitted in a subset of REs in PRBs used for an associated ePDCCH transmission.
FIG. 6 illustrates a conventional structure for DMRS REs in a PRB associated with a PDSCH.
Referring to FIG. 6, DMRS REs 610 are placed in a PRB. For two NodeB transmitter antenna ports, a DMRS transmission from a first antenna port is assumed to apply an Orthogonal Covering Code (OCC) of {1, 1} over two DMRS REs located in a same frequency position and are successive in the time domain, while a DMRS transmission from a second antenna port is assumed to apply an OCC of {1, −1}. A UE receiver estimates a channel experienced by a signal from each NodeB transmitter antenna port by removing a respective OCC.
Several aspects for a combined cPDCCH and ePDCCH operation in FIG. 5 still need to be defined in order to provide a functional design. One aspect is a process for a UE to detect cPDCCHs and ePDCCHs. To avoid increasing a UE decoding complexity and a probability that a UE incorrectly assumes a cPDCCH or an ePDCCH as intended for it (i.e., a false CRC check), it is desirable that a total number of respective decoding operations is substantially the same as when a UE does not monitor any ePDCCH transmissions (for example, as illustrated in FIG. 1).
Another aspect is that for ePDCCH reception based on a DMRS, a desired reliability of channel estimate should be ensured especially for UEs experiencing low DL SINR and requiring highly reliable ePDCCH receptions. Unlike the case with a CRS, time-domain interpolation across different DL subframes may not be possible with a DMRS and, as an ePDCCH transmission is assumed to be either in one PRB or in two or more non-adjacent PRBs, frequency-domain interpolation across different PRBs may also be impossible.
Another aspect is a PUCCH resource determination for a HARQ-ACK signal transmission in response to a reception of TBs conveyed in a PDSCH scheduled by a respective DL SA transmitted in an ePDCCH.
Therefore, there is a need for an ePDCCH decoding process at a UE in a communication system supporting both cPDCCHs and ePDCCHs.
There is another need for a UE to determine a PUCCH resource for HARQ-ACK signal transmission in response to a reception of data TBs conveyed in a PDSCH scheduled by a respective DL SA transmitted in an ePDCCH.
Further, there is another need to enhance the reliability of a channel estimate provided by the DMRS in a PRB conveying ePDCCH beyond the one obtained in a PRB conveying PDSCH.